The present invention relates to a method for machining a glass substrate in which a groove-like concave portion is formed in a surface of the glass substrate, and particularly to a method for forming a V-shaped groove in the surface of the glass substrate by laser ablation.
A groove-like concave portion formed in a substrate is used as a member for retaining an optical device such as an optical fiber, a rod lens or the like, or as an optical device such as a diffraction grating or the like. When the groove formed in the substrate is used as a member for retaining an optical fiber or the like, it is important that a section of the groove perpendicular to a lengthwise direction of the groove is V-shaped. In the case where the groove section is U-shaped or rectangularly shaped, retention of the optical fiber or the like in the groove is performed not with line contact but with surface contact so that the size of the groove section is required to be coincident with the diameter of the optical fiber or the like with high accuracy. If the size of the groove section varies, the optical fiber or the like cannot be fixed into the groove surely so that the optical fiber or the like is moved inside the groove. Because the retention of the optical fiber or the like is performed for the purpose of aligning the optical axis of the optical fiber with that of another optical device, such movement causes a problem. On the other hand, in the case where the groove is V-shaped, the groove section is inclined linearly so that the optical fiber or the like can be retained with line contact between the opposite wall surfaces. Accordingly, there is no fear that the optical fiber or the like may be moved inside the groove.
Most of the V-shaped grooves used for the above-mentioned purpose are formed by chemical etching. The speed of etching varies in accordance with crystal orientation of the crystalline substrate such a silicon substrate or the like. When, for example, single crystal silicon is etched with an alkaline etching solution, the etching speed for a face (100) or a face (110) is higher than that for a face (111) so that a shape constituted only by a (111) crystal face can be formed (e.g. see “LSI Handbook”, Institute of Electronics and Communication Engineers of Japan, the OHM sha Ltd.)
Such an etching method is called anisotropic etching. For anisotropic etching of the silicon crystal face, there may be used an alkaline solution such as KOH, N2H4 (hydrazine), NH2 (CH2)2NH2 (ethylenediamine), NH4OH (aqueous ammonia), or the like. Si may be removed in the form of SiO2 (OH)2− with OH− ions in the alkaline solution. Alcohol such as CH3.CHOH.CH3, C6H4 (OH)2 (pyrocatechol) or the like is often used as a buffer. It is considered that the buffer prevents OH− ions from being adsorbed onto the Si surface, reduces the etching speed easily controllably and changes plane-azimuth dependence.
When an Si (100) wafer surface is etched with the aforementioned etching solution in the condition that a mask such as a photo resist or the like having apertures of stripe-like patterns each having a constant width is provided on the Si (100) wafer surface, V-shaped grooves are formed. Etching is advanced while an angle between opposite side surfaces of each of the V-shaped grooves is kept at 54.7 degrees. The reaction substantially stops at a depth determined by a pattern width of the mask.
Because a crystallographically determined shape can be formed according to anisotropic etching, accurate machining can be performed compared with any background-art method. According to this anisotropic etching method, a V-shaped groove having any size can be formed by simply changing the pattern width. In addition, the method is an etching process in which a large quantity of grooves can be formed simultaneously. Hence, the method is advantageous in low cost of production when a large quantity of grooves each having the same shape are produced.
The method using anisotropic etching is, however, applicable only to a crystalline substrate such as a single-crystalline silicon substrate or the like as the material of the substrate. Further, in the method, the angle of the V-shape is determined crystallographically uniquely, so that the angle cannot be adjusted. These facts cause the following problem.
The higher performance is required of the optical device, the more serious problem is characteristic variation owing to the temperature of an optical system. This problem is caused by the expansion/contraction of each of optical devices constituting an optical system in accordance with the change of the temperature and is caused by the change of the optical path length in accordance with the change of the refractive index. Therefore, if a material having characteristic to cancel the change of the optical length of the optical device in accordance with the temperature change is used as the retaining member, the temperature change can be reduced on the whole of the optical system.
In the case of the retaining member used for aligning the optical axes of optical devices, the change of the temperature has influence on optical axis displacement due to expansion/contraction. On this occasion, a material having small expansion/contraction with respect to the temperature change may be used. For example, the thermal expansion coefficient of silicon is, however, about 25×10−7° C.−1, so that it is difficult to select a material small in expansion/contraction from the aforementioned crystal materials used for anisotropic etching. On the contrary, materials small in thermal expansion coefficient such as quartz glass (5.5×10−7° C.−1) are present as glass materials. Further, a material called zero-thermal-expansion glass is known as a material having a thermal expansion coefficient smaller than that of quartz glass. In addition, in the case where an optical device on the retaining member has a positive thermal expansion coefficient, glass having a negative thermal expansion coefficient may be selected as the material for the retaining member so that the glass can cancel the thermal expansion of the whole system. As described above, as a retaining member which suppresses the change in characteristic of the optical device owing to the change of the temperature, a glass material which is an amorphous material can be selected in a wider selection range than that of a crystal material.
As the method for forming the V-shaped groove in the glass substrate, cutting by a dicing saw may be used. In this method, the substrate is cut while a blade finished accurately is rotated at a high speed. Accordingly, this method is applicable to a wide range of substrate materials, and has a feature that the angle of the V-shape and the width and depth of the groove can be changed desirably by exchanging the blade edge.
In the method of cutting by a dicing saw, however, the blade edge used for cutting wears out so quickly that one blade edge can form only several V-shaped grooves. Therefore, there occurs a cost problem. Further, because the blade edge needs to be exchanged into a new one whenever every several grooves are cut, it is difficult to keep the interval between adjacent grooves with high dimensional accuracy. In addition, because the size allowed to machine the blade edge is not small than 50 μm, there is a problem that the width of the V-shaped groove obtained thus is limited to be not small than 50 μm.